1) Field of Invention
The invention relates to (1) a phase shift mask capable of transferring patterns with an improved resolution upon placing phase differentials between exposure light rays that pass through the mask, (2) a phase shift mask blank as a material for the phase shift mask, and (3) a manufacturing method for the phase shift mask. More particularly, the present invention is directed to a half tone type phase shift mask, and blank and manufacturing method for the mask.
2) Related Arts
It has been recently determined that the mere increase of the numerical aperture of an exposure apparatus' lens or the mere reduction of wavelength of an exposure apparatus' beam cannot improve practical resolution for lithography, since two key factors, rendition of higher resolution and assurance of depth of focus, required for lithography are in a trade-off relationship.
As a result, phase shift lithography is getting attention as the next generation of lithography. Phase shift lithography is a method to improve photo lithography's resolution by changing only masks without changing any optical system. It can grossly improve the resolution in using interference between transmission light rays upon placing phase differentials between exposure light rays that pass through the photo masks. Such a phase shift mask is a mask in common possession of light intensity information and phase information. It is categorized in several types such as, so-called Levenson, auxiliary pattern, self-alignment (edge enhancement), or the like. These phase shift masks have a more complicated structure than those of conventional photo masks which possess only light intensity information, and require a higher level of technology with regard to manufacturing.
As one of such phase shift masks, a phase shift mask referred as to a half tone type phase shift mask has been developed these days. In such a half tone phase shift mask, the light translucent portion (photo semi-transmission portion) produces a light shield function in which the exposure light is substantially shielded and a phase shift function in which the phase of the light is shifted (ordinarily reversed). Therefore, the half tone phase shift mask, without the necessity to separately form a light-shield film pattern and a phase shift film pattern, has a feature enabling the structure of the mask to be made simple and the manufacturing process of the mask easier.
A typical conventional half tone type phase shift mask is shown in FIG. 1. The half tone type phase shift mask has on a transparent substrate 1 a mask pattern, which is as shown in FIG. 1(a) composed of a light transmitting portion (a photo transmission portion, the exposure areas of the transparent substrate) 2 through which light with intensity capable of substantially contributing to the exposure process passes, and a light translucent portion (light shield and phase shift portion) 3 through which light with intensity unable to substantially contribute to the exposure process passes. The phase of light passing through the light translucent portion is shifted, and the phase of the light that has passed through the light translucent portion thereby has a substantially reverse relationship with the phase of light that has passed through the light transmitting portion (see FIG. 1(b)). Lights passing through boundaries or their vicinity between the light translucent portion and the light transmitting portion diffracts so as to encroach one another due to a diffraction phenomenon and cancel out one another. The light thereby improves the contrast at the boundaries, or the resolution, while the light intensity around the boundaries is nearly nullified.
Meanwhile, the light translucent portion of the half tone type phase shift mask described above must have the optimum values for the transmittancy or transmission rate and the phase shift amount. It has been known that such optimum values in a phase shift mask can be realized by a single layer of the light transmitting portion. Such a phase shift mask has a light transmitting portion made of a thin film of, as a material or essentially all of a material, metals such as molybdenum, silicon, and oxygen, in particular molybdenum silicides, or more specifically, oxides of molybdenum silicon ("MoSiO system material"), or nitride oxides of molybdenum silicon ("MoSiON system material"). With these materials, transmittancy can be controlled by selecting the amount of the oxygen or the nitrogen and oxygen, while the thickness of the thin film can control the phase shift amount. When formed of these materials, the light transmitting portion can be structured as a single layer film made of a single kind material, thereby simplifying the film formation process, in comparison to a case where it is made of multilayer film made of multiple materials. The manufacturing process is simplified since a single etchant can be used.
However, the molybdenum silicide film or the molybdenum silicide nitride oxide film, as elements composing the light translucent portion of the phase shift mask is weak against acid, such as sulfuric acid, used for a primer or rinse fluid for rinsing during mask manufacturing or when masks are used. In particular, where the transmission rate and phase shift amount of the light translucent portion is set for a KrF excimer laser beam (248 nanometers), the attenuation constant (K) must be small, and to realize a small attenuation constant, the degree of oxidation or nitriding oxidation has to be increased sufficiently. If the oxidation degree is so increased, acid resistance of the light translucent portion is significantly lowered, thereby raising a problem that the transmission rate and phase shift amount thus set may have deviated.
When a film for phase shift mask blanks is formed, oxide tends to deposit on target surfaces, especially on non-erosion areas as the degree of oxidation or nitriding oxidation increases, and makes electrical discharge of the film unstable. Consequently, the transmission rate and phase shift amount is poorly controlled, thereby raising a problem that the blanks are formed with defects. Moreover, since the relationship between the composition of the film and film characteristics such as acid resistance, photo resistance, conductivity, deflection rate (thickness), transmission rate, selectivity of etching, etc. remains unsolved, the optimum film characteristics cannot be obtained in light of the manufacturing process or the like even if the required optimum values for both the transmission rate and the phase shift amount are obtained at the stage of blanks. Therefore those values deviate from the designed or set values through experiencing the manufacturing process, resulting in inability to form the optimum phase shift masks.